Ozone, which chemically is three oxygen atoms joined into a molecule (formula O.sub.2), is a highly reactive gas and a very strong oxidizer because it readily splits into a stable atmospheric oxygen molecules (formula O.sub.2) and a free single oxygen atom. Ozone has many and varied uses: it kills bacteria and viruses, purifies water, deodorizes carpets, bleaches fabrics, and so on.
In many applications there has been a need for a strong draft of ozonated air, of up to 100 cubic feet of air per minutes, from a compact ozone generator. However, the existing technology has not met this need for a compact, simple ozone generator with a high air-flow rate.
Ozone generators for ozonating air are of two basic types; ultraviolet and corona.
The ultraviolet type employs gas-discharge lamps which emit bright ultraviolet light (also called "UV" or "black light" lamps), which breaks up atmospheric oxygen and thus creates ozone. Because the UV lamps are bulky, an ultraviolet generator cannot be very compact. The lamps are also fragile and expensive and can burn out.
The corona or Siemens type creates ozone by subjecting air to a very high electric field. Typically the strong electric field is supplied from a transformer, which converts ordinary line current at 115 volts into alternating current (AC) at about 6000 volts, sufficient to break up oxygen molecules. The high-voltage coil of the transformer is connected to two electrodes so that one goes positive when the other is negative. The two electrodes form a sort of capacitor, and usually comprises two parallel plates, concentric cylinders, or some other geometry which allows for a constant distance between the electrodes. A dielectric (insulating) material, typically plastic or ceramic, is often included in between the two metal electrodes; however, it does not fill the gap completely because air to be ozonated must circulate through. The dielectric provides a solid insulator to prevent shorting between the electrodes and also intensifiers the electric field.
Ozone generators create substantial amounts of heat, and indeed heat build-up is a basic limited factor in ozone generators. To produce a great deal of ozone requires a large volume of air subjected to strong electric fields. Under these conditions, heat builds up quickly inside the dielectric material because of the strong alternating electric field to which it is subjected. This heating of the dielectric is analogous to heating in a microwave oven. The dielectrics can suffer a shortened life, melt, crack, and so on. Air temperatures above 130 degrees Fahrenheit also interfere with ozone production.
(The metal electrodes do not generate so much heat internally as dielectrics, and are better able to withstand elevated temperatures; of course, the air between the electrodes can be heated without damage.)
Previous workers in the field above have devoted considerable energy to the problem of removing heat from the dielectric. Often the cooling is quite elaborate. For example, U.S. Pat. No. 3,766,051 discloses concentric tubes electrodes with coolant fluid both inside the inner electrode and outside the outer electrode. Another example is U.S. Pat. No. 4,960,570 issued to Mechtersheimer shows an arrangement with cooling fluid circulating outside electrode plates (labeled 1 and 2) and also inside electrode tubes (3) held in a row between the plates.
It is much less expensive, and simpler, to cool the dielectric with air. However, if only air is used then the problem of heat removal is made more difficult, because air, compared to a liquid like water, is a thermal insulator and cannot hold as much heat per unit volume. One need only imagine standing in 60-degree air while wearing a swim suit and standing in 60-degree water while dressed the same. Water removes heat from the body about 3600 times faster than air does.
Moreover, the heat problem becomes worse as the ozone generating unit is made smaller and more compact. Clearly, to produce ozone at a certain rate, heat must also be generated at a certain rate; but the smaller the unit, the less the internal and external surface area is from which the heat can be lost, and the higher the temperature of the unit becomes.
Thus, the problem of producing an ozone generator which is both compact and air-cooled is double difficult. Prior workers have not solved it.
Batchelor, in U.S. Pat. No. 5,008,087, discusses the heat problem and attempts to solve it by using reversing air flow. Air is injected into the annular gap between a round inner electrode (12) and the inside of a dielectric tube (16). At the far end the air is forced to reverse direction and flow between the outside of the dielectric tube and a tubular outer electrode (14). This is claimed to reduce the temperature differential between the inside and outside surfaces of the dielectric tube, reducing the risk of heat cracking (at column 2, lines 25-54). The diameter of the inner electrode is given as 1.25 inches.
Aside from the obvious fact that the outside surface of the dielectric tube is being "cooled" with already-heated air, reversing the flow increases air resistance and makes the design more bulky, since the air is removed from the same end at which it is injected. The air needs to turn many corners, the flow is not straight-through, and the diameter of the unit is increased.
A "Compact Ozone Generator" is described by Coate in U.S. Pat. No. 5,503,809. An inner dielectric tube (18) and an outer dielectric tube (16) have electrodes in between them, within, and without. Air flows through the inner tube, reverses, and flows out in the annular space between the tubes. Unlike Batchelor, Coate does not attribute cooling properties to the reverse flow, and instead advises (column 3, line 49) that the feed air be chilled to cool the unit. The refrigeration unit required for this will naturally obviate any advantages of Coate's "compact" design.
The conventional technology has not provided an effective solution to the heat problem in air-cooled ozone generators. It has particularly not provided a compact, large output, air-cooled ozone generator which can eliminate heat from the dielectrics at the required rate.